The presence of environmental toxicants in human breast milk is an important concern because of the potential health effects of these chemicals to the breast-fed infant. Lipophilic toxicants are preferentially concentrated in high-fat breast milk, and they include several different classes of synthetic organic pollutants such as polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polybrominated diphenylethers (PBDEs) and persistent pesticides. The body burden of these toxicants in breast-fed infants is more than an order of magnitude higher than it is in adults, and elevated levels in adults have been linked to several disorders of the reproductive, behavioral, and central nervous systems. The continued efforts of environmental scientists to measure toxicants in breast milk are an important factor in the determination of the true contribution of these chemicals to the health of the infant population. There is a need to develop new analytical screening strategies that will monitor the levels of known environmental pollutants as well as identify additional toxicants in breast milk. Current screening methods employ sample extraction protocols that are time-consuming, and the analysis of samples is limited to the list of known toxicants and therefore not capable of detecting new or unknown contaminants. This project is intended to develop an improved method based on the combined emerging technologies of stir bar sorptive extraction (SBSE) and comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GCxGC/TOF MS). The specific aims are to: (1) develop a high-throughput sample preparation protocol based on the stir-bar sorptive extraction (SBSE) technique; (2) develop a high-performance sample analysis protocol based on the emerging technology of comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GCxGC/TOF MS). The key advantages of GCxGC/TOF MS are enhanced resolution power, analytical throughput and sensitivity over conventional GC; and (3) Validate the new method through a pilot study that will test the overall selectivity, sensitivity and analytical throughput of the GCxGC/TOF MS method when compared to the current state-of-the-art.